Cryogenic refrigerating means



Jam-30, 1962- D. D. EVERS CRYOGENIC REFRIGERATING MEANS mgln I/ 3,018,643 CRYOGENIC REFRGERATING MEANS Dundred D. Evers, Fort Washington, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Filed Sept. 15, 1959, Ser. No. 840,786 2 Claims. (Cl. 62-467) This invention relates generally to the field of cryogenio engineering and more particularly to improved -refrigerative means of the gas-liquifying type.

Nearly all present day cryogenic temperature processes employ some type of gas-liquifying cooling system. One particularly effective method of liquifying gases is that of employing a gas throttling process embodying regenerative cooling of the incoming gas by the expansion-cooled outgoing gas.`

Low temperature regulators of this type, herein termed cryostats, have the advantage of being free of moving parts susceptible to low temperature malfunctioning in addition to being of extreme structural compactness. Because of the growing demand, particularly by the Military for a compact, self-contained refrigerating package for use in missile, aerial reconnaisance and other related applications, this type of cooling apparatus has become of increasing importance. While cryostats operating on the expansion or Joule-Thomson cooling principle exist in a multitude of forms, they typically comprise a tightly wound coi-l of high-heat-conductivity tubing housed in an insulating sheath and adapted to transport gaseous media under high pressure to an expansion orifice or throttling valve, through which the gas is expanded to approximately atmospheric pressure. The Joule-Thomson cooling resulting from expansion causes a lowering of temperature, and the cooled expanded gas is constrained to pass back over the incoming passages of the cryostat, or heat exchanger, to cool the incoming high-pressure stream. The temperature at the valve is progressively lowered until gas liquifaction temperatures are reached.

Despite the advantages inherent in cryostats of this type, however, they have found only limited acceptance because of their tendency toward erratic and unreliable operation and because they can only be safely used in situations where continuous and reliable performance is of secondary concern. It should be noted however that these deficiencies can be overcome by using highly purified gas but this approach, when available, is prohibitive from a cost standpoint.

Accordingly it is a general object of the present invention to provide a gas liquifaction refrigerative system which overcomes the limitations of the prior art.

Another and more specific object of the invention is to provide a counterilow cryostat operating on the Joule- Thomson cooling principle having improved performance from the standpoint of reliable use over extended periods i of time.

A still further object of this invention is to obviate the necessity of using gases of unusually high purity thereby to provide a simpler less expensive cryogenic cooling system.

In achievement of the foregoing general objectives I utilize a cryostat of the type employing regenerative Joule-Thomson cooling provided with a plural tube inlet through which gas passes in parallel flow relation to a common outlet orifice and including means for providing a differential heat exchange relation between said tubes and the stream of expanded gas.

These and other objects ywithin contemplation will be apparent by reference to the accompanying detailed description and drawings, in which:

FIGURE l illustrates a cooling assembly constructed in accordance with the present invention, said assembly being shown, for exemplary purposes, yas used in refrigerating an infrared detector cell;

FIGURE 2 is a partially sectionalized enlargement of the novel temperature regulating element or cryostat shown in FIGURE l; and

FIGURE 3 shows a modified form of emission orice designed for use in a dual-tube cryostat.

The cooling assembly 9 shown in FIGURE l comprises an insulating jacket 10 encasing a dual tube regenerative cryostat 11, the latter element constituting a preferred embodiment of the present invention. The jacket 10 is of the conventional Dewar construction and consists of an outer cylindrical glass portion 12 joined through a cold-welded flange assembly 13'to the outwardly disposed overhanging skirt 14- of an inwardly depending, re-entrant cylindrical sleeve 15 spaced from and somewhat shorter than the outside cylindrical portion 12 of jacket 10. A metal cup 16 which is typically composed of a Kovar alloy is sealed to the inwardly disposed depending end of sleeve 15. Afl'lxed, as by soldering, to the base of this cup, in high heat-conductive relation therewith, is an infrared detector cell 17. Electrical accessibility is afforded to this cell by means of connectors 1S forming a connecting bridge between this cell and electrically conductive strips, eg. platinum ribbons 19, fused to outwardly presented lower surface portions of the re-entrant sleeve 15. These strips connect with terminals 20 to facilitate connection of the cell to external circuitry. To protect crystal 17 from atmospheric contaminants, while permitting its free infrared irradiation, an infrared transmissive window 21, cornposed for example of sapphire, hermetically seals the open end of the external cylinder 12.

Infrared detectors of this general type are known, being useful for a variety `of purposes certain of which are military, and to provide a detector of requisite sensitivity, namely, one having optimum spectral response and one capable of detecting small temperature differentials, requires that the cell be maintained at cryogenic temperatures. When using, for example, an N-type golddoped germanium crystal for infrared detection, optimum results are obtained by cooling the crystal to liquid nitrogen temperatures i.e. to a temperature of approximately 78 K.

In operating this cell a body of liquid nitrogen 30 is produced and replenished within metal cup 16 by the novel dual-tube regenerative cryostat of the present invention. In the form shown, cryostat 11, see FIGURE 2, comprises a plastic cylindrical mandrel 31 upon which is helically wound coil of metal tubing 32 used in novel combination with and in heat exchange relation with an unfinned tube 34, the tubes being connected to provide a parallel flow circuit for the passage of high pressure gas to a common orifice 35. As an illustrative example, an embodiment found to produce highly satisfactory results comprised tubing, which may be of the type used in the lfabrication of hypodermic needles, having 5 mil wall thickness -with an I D. of l0 mils, the diameter of the mandrel 31 being such that the tubing tits snugly within the cavity defined by the re-entrant sleeve portion 15. In this connection the finned tubing has an extended length of 8 and the unfinned tubing an extended length of l2" with the compacted assembled length of the overall system being about 11A. Both tubes terminate in a common exit chamber 36 closed by the apertured cap 37 seen in section in FIGURE 2. Gas is discharged from this orifice to atmospheric pressure. In the embodiment under consideration, the orifice is approximately two mils in diameter and is positioned in overlying adjacency to the metal cup 16. Nitrogen gas at a pressure of 1200 p.s.i. and at approximately room temperature is applied to the other end of the tubing through hydraulic coupling 38. The Joule-Thomson cooling upon expansion causes a lowering of temperature and the cooled expanded gas is constrained by the insulating jacket to pass back over the gas conducting tubing where it cools the incoming high-pressure stream. By this process of regenerative cooling the temperature at the valve is progressively lowered until the liquifaction temperature of nitrogen (78 K.) is reached.

The body 30 of liquid nitrogen thus produced within the Kovar cup 16 maintains crystal 17 at a temperature approximately equal to that of liquid nitrogen. As a result, the sensitivity of the system, particularly to longwave infrared radiation is very much greater than that obtainable from systems wherein the infrared detector must operate at room temperatures. The cooling assembly 9, as will be understood, is susceptible to numerous variations without departing from the scope of the invention, the embodiment shown being merely illustrative.

Prior art cryostats which did not include the overlying wrapping of unfinned tubing 34, give rise to very erratic operation, often only running at liquifaction temperatures for a fraction of a minute before flow malfunctioning interrupts operation. I have found, however, that by applying a second parallel ow path in conjunction with the finned portion of the tubing that substantially uninterrupted operation of the cryostat can be insured for several hours and in some instances for periods as long as several days. Thus by the simple expedient of using a dual-tube cryostat having tubes maintained in differential heat exchange relation with the effluent gas stream, the systems erratic operation is eliminated.

While the theory underlying the operation of this invention has not thus far been proven, the following theory is believed explantory of the systems dramatically improved performance. Gases conventionally employed in gas-liquifying processes contain trace amounts of contaminants. Nitrogen, the gas used in the illustrated application of the invention, is normally obtained by the fractionation of liquid air and invariably contains trace amounts of water vapor, carbon dioxide and varying quantities of the rare gases. As regenerative cooling occurs these contaminants or impurities are believed to freeze out obstructing the iiow of gas through the tube. While the emission orifice is considerably smaller than the I.D. of the tube it has been observed that fiow obstruction is generally caused by clogging of the tube conduit.

It is believed that when malfunctioning occurs as a result of freeze-out, the warmer tube serves to thaw-out the jammed colder tube while concurrently maintaining fluid flow of the refrigerating coolant. This effect is brought about in the illustrated example by the differential heat exchange resulting from finning only one of the gas-conducting tubes.

It has been observed that in a single tube cryostat employing a 10 mil LD. tube communicating with a 2 mil orifice and operating under a fixed inlet pressure of 1200 p.s.i., that the pressure drop along a helically Wound tube having the length mentioned is approximately 600 lbs. The exit pressure at the orifice is accordingly reduced to about 600 p.s.i. When the second tube is added, in the manner illustrated, the observed pressure drop across the parallel lines is reduced to a value of approximately 200 p.s.i. resulting in a higher outlet pressure of 1000 p.s.i. This accounts for the increased efficiency of the inventive system from a standpoint of the time needed to reach liquifaction temperatures since the temperature reducting resulting from Joule-Thomson cooling is proportional to the pressure drop. The dualtube system when compared with a single finned tube cryostat of the same dimensions produced liquid nitrogen after only about 2 minutes of operation as compared to a 6 minute period required for a single-tube liquifier.

Moreover, in the event of freeze-up the dual tube arrangement provides a self-regulating system which automatically acts to clear the clogged line. This is brought about by the fact that should the colder line experience ow malfunctioning, there is an immediate reduction in the outlet pressure since in effect the system temporarily reverts to a single tube cryostat which is observed to produce a greater pressure drop across the line than the dual tube system. Since Joule-Thomson cooling is proportional to the pressure drop the temperature of the escaping gas immediately rises. This warm air is then recirculated over the tubing warming the infiowing gas particularly the gas confined within the finned tubing which has a greater heat transfer rate, raising its temperature and removing the occlusion. Simultaneously the rate of ow through the orifice, because of a decrease in output pressure, is diminished providing an increased heat transfer rate between tubes. Both these effects act to prevent freeze-up and are believed to account for the unprecedented reliability of this unique gas liquifying system.

A constructional refinement further improving reliability of the system is the use of an outlet chamber 36 common to both tubes closed by a separately formed apertured cap 37. This construction shown most clearly in FIGURE 2, enables the orifice 3S to be machined separate and apart from the tubing thereby allowing more accurate dimensioning and the removal of burrs before assembly of cap and tubing. Connection to the high pressure gas line is made through a ve micron filter 40 which removes `articulate matter from the gas which might tend to clog the orifice or nucleate the formation of crystals.

FIGURE 3 illustrates an alternative outlet construction which consists merely of providing an orifice 41 in the connecting loop between tubes. Conventional pinched-off construction can also be used, but the preferred form is that shown in FIGURE 2.

In summary, I have found that improved reliability of cryogenic refrigerating means employing gas-liquifaction of the general type illustrated can be obtained by the novel expedient of providing dual flow paths for the passage of gas tothe expansion orice maintained at different operating temperatures. A satisfactory but by no means exclusive technique for obtaining differential cooling of these tubes is by the selective use of finning in the manner shown coupled with regenerative cooling.

While I have described my invention by means of specific examples and specific embodiments, I do not wish to be limited thereto, for obvious modifications which do not depart from the scope of the invention will occur to those skilled in the art.

I claim:

1. A gas-liquifying cryostat comprising: a pair of tubes in mutual heat exchange relation constructed to transport gaseous media under high pressure to a common outlet through which said media is expansively cooled;`

means for regeneratively cooling the incoming gas stream by counterflowing gaseous portions of the expanded fluid over said tubes to reduce the temperature of said incoming gas to a value permitting liquifaction thereof; and means providing for a substantial differential rate of heat transfer between the outgoing gas stream and each of said tubes to permit operation of said tubes at different temperature levels.

2. A cryogenic gas liquifier comprising a pair of tubes in mutual heat exchange relation for flowing gaseous media under high pressure to a separately formed chamber closed by an apertured Wall through which aperture said gaseous media expands to approximately atmospheric pressure to effect cooling thereof; means constraining the cooled expanded media to pass back over said tubes -to reduce the temperature of said incoming gas progressively to fa value permitting liquifaction thereof; and means providing for a differential rate of heat transfer between the outgoing gas stream and each of said tubes to permit operation of said tubes at different temperature levels.

References Cited in the tile of this patent UNITED STATES PATENTS Swart Aug. 25, 1936 Streeter July 2l, 1959 OTHER REFERENCES 

